The 3rd-Generation Partnership Project (3GPP) has initiated a study of potential improvements to be included in a further advanced version of the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), commonly known as the Long-Term Evolution (LTE) system. (The anticipated improved version is commonly referred to as LTE-Advanced.) One of the several objectives of the study is that there should be a manyfold increase in spectral efficiency and system/user throughput, especially at cell edges, in comparison with legacy systems. Legacy systems such as WCDMA and E-UTRAN, which uses OFDMA, reuse transmission frequencies in adjacent cells, allowing efficient use of sparse spectrum resources. However, this also leads to inter-cell interference, especially at cell edges, which is generally mitigated by employing advanced receivers at the base stations and mobile terminals. In an advanced E-UTRAN system the inter-cell interference is expected to be restricted by architectural means in addition to traditional means based on advanced receivers. One such arrangement to be employed is called a distributed antenna system (DAS).
In a wireless system that employs a distributed antenna system (DAS), each cell in a network of several cells includes two or more spatially separated antenna nodes, remote antenna units, base station sites, or so-called sub-base stations, connected to a common source via a transport network. This group of spatially distributed nodes, or subcells, together provide wireless service to mobile terminals within the boundaries of a specific geographic area, or cell. Each of these nodes, or sub-base stations, within a DAS cell can be passive amplifiers, or each may contain full signal processing capability (i.e., a transceiver). All sites within a particular cell are tightly synchronized.
Another commonly termed used in the current literature for DAS is coordinated multipoint transmission/reception (CoMP). However, in the discussion that follows, the term DAS will generally be used, with the understanding that this term is intended to refer broadly to systems utilizing coordinated multipoint transmission and reception from multiple subcells in a cell, including those systems currently proposed within 3GPP. Furthermore, although various sites within a DAS cell may have slightly varying levels of functionality, all of these sites within a DAS cell will generally be referred to as sub-base stations in the discussion that follows, while the area covered by each sub-base station will be referred to as a subcell. Similarly, the area covered by a coordinated group of DAS subcells will be referred to as a DAS cell.
FIG. 1 illustrates the basic concept of a DAS-based architecture, showing multiple DAS cells 110, each of which includes several DAS sub-base stations 120. The area served by a given DAS sub-base station 120, i.e., the subcell, is smaller than the area of the entire DAS cell 110. Thus, the general idea in a wireless system using DAS is to split the transmitted power among several sites separated in space so as to provide coverage over the same area as a conventional, single-base station cell, but with reduced transmission power levels and improved reliability. This approach leads to reduction in the inter-cell (and inter-site) interference.
Typically, the UE receives signals from more than one site in a DAS cell 110, i.e., multiple subcells may simultaneously serve a user. Since the sub-base stations within a DAS cell 110 are all coordinated, i.e., synchronized, the mobile terminal can receive the transmitted data transparently and coherently.
Those skilled in the art will appreciate that the introduction of DAS will impact many receiver processes, including cell search procedures and the periodic signal measurements required for performing cell reselection and handover. Accordingly, suitable procedures for measuring downlink signals and reporting the resulting measurement information are needed.